Actually, that is a question I've given quite a bit of thought to, not because I think I can reach an answer, but because I want to consider the major factors and constraints.

I think this planet can indefinitely, comfortably, support about a billion people (without assuming quantum leaps over our present technology, like FTL, replicators, or antimatter power reactors).

I further assume we can do this and retain an advanced technology and a democratic form of governance, civil rights and personal freedoms under some blend of capitalism and socialism. I also expect that we can learn to manage our economy and technology if we don't simply turn it over to the business community to do anything they damn well please with it and hang the consequences. I am also confident that our economy can be self-sustaining (it doesn't require an exponentially increasing amount of external energy and resources to keep it functioning), or that continuous undisciplined growth must be a prerequisite for prosperity. I expect that our standard of living can be comparable to the average contemporary Western European city dweller. Millions of people actually enjoy living in Manhattan or Paris or San Francisco, and there will still be room for those who prefer to live in small towns or near the land. Just because I reject American automotive suburbia and consumerism doesn't mean we all have to live in barracks and wear uniforms, or return to an eighteenth century pastoral existence. We can have our cake and eat it too, as long as no one gets more than his share.

I expect about half the world's developable land area will be held in reserve as either wilderness or protected parklands (perhaps not in its original wild state but still "undeveloped"), and the oceans will be essentially a carefully managed wilderness (fisheries productivity will be optimized so that biological stocks are self-renewable forever). The earth's remaining land area will either be devoted to intensive renewable agriculture or other industrial or extractive uses, probably on a rotating basis so that the land is not systematically mined of all carrying capacity and then allowed to become toxic waste. Consumption of non-renewable resources will be held to a minimum so that technological innovation or renewable resources can be gradually substituted for those materials that do run out.

Keeping a world like this operating will require full education and full employment for the entire population, as well as a strong system of social services and welfare. Personal income differences for the financial extremes of the population will be on the order of between one or two orders of magnitude at most, with the income distributed normally throughout the population: few poor, few rich, most in the middle. And it will depend entirely on their productivity, not on the productivity they control. Excess wealth over and above needed to maintain the commonwealth will be directed into the space program.

Foolish utopianism? Certainly not. If you look at our history, we are better than halfway there already on large parts of the northern hemisphere, and only an exploding population and human greed threatens us from achieving it on a worldwide basis. The goals I have outlined are already in sight! And this has been accomplished in spite of the resistance and reaction of those who tell us it is an unachievable fantasy. We have made enormous progress towards this ideal in the last century, in fact, the world I have described is nowhere near as "impossible" as our present world was to the beginning of the twentieth century. Industrialization and technology are not random, physical forces, they are strictly man-made processes and they can be managed and optimized. We can have them work for us, not the other way round.

You may say I'm a dreamer, but I'm not the only one.